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Fascinating Phoebe

Cassini's close look at the moon last year prompts planetary scientsts to conclude Phoebe was born in the Kuiper Belt.Robert Adler

Published: Monday, May 16, 2005

The sharply-defined crater just above the center of this Cassini image shows layers of alternating bright and dark material. Cassini was 8,314 miles (13,377 kilometers) from Phoebe when this image was taken.

NASA / JPL / CICLOPS

May 16, 2005Saturn's outermost moon, Phoebe, remains a mystery. Its eccentric, tilted orbit, opposite Saturn's spin, suggests it didn't form alongside Saturn's other satellites. Two new studies using the Cassini-Huygens probe have essentially confirmed this theory. The independent studies tag the Kuiper Belt, on the fringes of the solar system, as Phoebe's likely birthplace, but the case is far from closed.

Near closest approach, Cassini captured this view of an 8 mile (13 kilometer) wide crater with a debris-covered floor. Radial streaks on the crater walls are due to ejacta fragments sliding down the slope. Boulders ranging in size from about 160 to 990 feet (50 to 300 meters) wide are also visible. They may have been excavated by large impacts, perhaps from some other region of Phoebe. There is no visible evidence for layering in this region, as on other parts of this moon.

NASA / JPL / CICLOPS

Just before inserting itself into orbit around Saturn, Cassini cruised to within 1,250 miles (2,000 kilometers) of Phoebe, providing astronomers with a unique chance to study the battered moonlet.

Torrence Johnson, at the Jet Propulsion Laboratory in Pasadena, California, and Jonathan Lunine, at the University of Arizona's Lunar and Planetary Laboratory, used deviations in Cassini's course caused by Phoebe's gravitation to calculate Phoebe's mass and density.

They found that the 140-mile (220 km) wide satellite weighs 1,630 kilograms per cubic meter. This clearly separates Phoebe from Saturn's other moons, which have an average density of just 1,300 kg/m3. "It's a pretty dense object, especially considering that it is likely to be somewhat porous," Lunine told Astronomy.

Phoebe's density closely matches that of two outer solar-system objects (after adjusting for the extent to which their greater mass compresses them) — Pluto, now firmly identified as a Kuiper Belt object (KBO), and Neptune's moon Triton, which likely started life in the Kuiper Belt before being captured by Neptune. Johnson and Lunine conclude that Phoebe appears to have "a composition identical to that of the only Kuiper Belt objects for which we possess densities."

Lunine's views gain support from a study of Phoebe's surface led by Roger Clark of the U.S. Geological Survey in Denver, Colorado. Using Cassini's Visible and Infrared Mapping Spectrometer, his team identified dozens of chemicals on Phoebe, including ice, carbon dioxide, iron- and carbon-based substances, and cyanide compounds. Phoebe's cyanide signature has been detected on just two other solar-system objects, Comet Borrelly and the dark face of Iapetus, Saturn's outermost large moon.

Brad Dalton of NASA's Ames Research Center at Moffett Field, California, thinks the density and spectroscopic studies together make a strong case for Phoebe's origin as a KBO. "While this is not incontrovertible proof … the independent findings of both groups give a very consistent picture," he says. "While I think the Clark et al. compositional analysis is very strong, the two sets of analyses taken together is what is really convincing."

"What is clear," Dalton says, "is that Phoebe falls into a class of objects that now populates the outermost reaches of the solar system, including the Kuiper Belt."

Clark remains cautious about Phoebe's origins, but he has no doubts about the value of the new Cassini findings. "The detection of carbon dioxide and the cyanide compounds were very unexpected," he says. "We didn't think Phoebe would be this exciting."